In recent years, it has become very common to use wind for the generation of electrical power. In order to do this, wind is captured by a set of blades of a wind power plant. The captured wind causes a shaft connected to the set of blades to rotate. The shaft is typically connected to a rotor of an electrical generator which is rotated in accordance with the rotation of the shaft, possibly at a multiple of the rotation speed of the shaft in case the rotor is connected to the shaft via a gearbox. The electrical generator converts the mechanical power provided by the wind in form of the rotation into electrical power which may be supplied to a power grid.
For various functions of a wind turbine generator, a determination of the rotation speed and/or the position of the rotor of the electrical generator is required, such as for stator flux control which allows controlling the magnitude of the electromagnetic power provided by the electrical generator.
The determination of the rotation speed and the position of the rotor of the electrical generator is typically carried out by a so-called encoder which may have an offset, e.g., determines the angular position of the rotor with a certain offset. Therefore, before any function for which a determination of the rotation speed and/or the position of the rotor is required can be started, which typically includes the start of actual power supply of the electrical generator to a power grid, offset encoder calibration has to be carried out, i.e. the encoder offset has to be determined by which the phase angle of the rotor output by the encoder is adjusted such that the adjusted phase angle is aligned with the actual phase angle of the rotor (e.g., a permanent magnet rotor).
A known method for encoder offset calibration from a single stator line voltage is based on zero crossing detection of the line voltages output by the electrical generator (i.e. a stator line voltage of the electrical generator). At each cycle when the line voltage is found to have changed from a negative value to a positive value, a zero crossing point is detected. At this time instant, the corresponding output by the encoder is captured and the offset is calculated. Similarly, a rotor flux may be computed based on peak value detection of a line voltage signal.
Since the zero crossing detection is prone to the noise, the accurate detection of the zero crossings for encoder offset calibration may be very difficult to achieve, even after some measures are used to improve it like linear interpolation, averaging etc.
Another issue is the calibration time. Since only one zero crossing point in one cycle (of the line voltage) is used for calibration, the processing time is long.
Hence, an objective of the present invention may be seen in providing a method for determining a rotor position of an electrical generator which may be used for encoder offset calibration and which is more accurate and requires less processing time.
Since the encoder is prone to failure in a WTG (wind turbine generator), a real time tracking of encoder health is required for reliable operation. It should be able to track encoder health real time in normal operation, and send the alarm in case of encoder failure and malfunction. An object of the present invention may also be seen in providing a method for this real time health tracking.